The present invention relates to pilot warning systems and, more particularly, to a calibration technique for a pilot warning system that employs time-of-arrival, amplitude or phase comparison to determine the direction to a source of electromagnetic radiation.
My prior U.S. Pat. Nos. 5,506,590 and 5,223,847 disclose a passive system for determining the location of an emitting radio source. The system measures the azimuth and elevation angles of arrival, with respect to the deck of an aircraft, of transponder signals emitted by airborne targets in which a pilot of an aircraft may be interested.
These systems in these prior patents, particularly the measurement of elevation angle, depend on relatively small differences in time of arrival of received signals to derive the required data.
Azimuth to a target aircraft is preferably determined by comparing the outputs of four separate receiver channels. A technique is disclosed wherein an array of four antennas surrounding a parasitic center element feed four separate receivers respectively. The center element is initially floating so that the four antennas are substantially omnidirectional. The base of the center element is grounded midway through a received pulse to increase the received intensity along a line from the center element to the one of the four antennas closest to the target bearing. The increase in received strength, although theoretically about a factor of 2, because of inevitable losses, is closer to a factor of about 1.8. The one of the receivers exhibiting such an increase in received signal strength beginning at the switching time indicates the azimuth of the emitting target. One skilled in the art will recognize that more than four azimuth angles can be detected using interpolation of signals on more than one receiver.
A calibration technique for the measurement device for elevation and azimuth angles is not disclosed in my prior patents.